On February 10, the Unix-based email server Exim released an update to address a heap buffer overflow vulnerability that can be used by an unauthenticated attacker to remotely execute arbitrary code. The flaw, assigned CVE-2018-6789, is noted to exist in all versions of Exim, prior to their latest release, 4.90.1, which means the attack surface potential is very wide. A quick search on Shodan yields more than 6 million results.

Vulnerability details

The vulnerability was originally discovered by DEVCORE, and details were published on their blog on March 6. The vulnerability is due to a flaw in the b64decode buffer length in the base64d() function. Due to an off-by-one calculation mistake, heap memory can be overwritten when parsing an invalid base64 string leading to critical data being overwritten.

As base64 decoding is a widely used function, and since the byte is user-controlled, this increases the ease of exploitation, which can be utilized for remote code execution.

Identifying affected systems

To detect systems affected by this critical flaw, Tenable has released Nessus® plugins for Tenable.io Vulnerability Management, SecurityCenter and Nessus Pro. Additionally, Tenable has released passive detection via Nessus Network Monitor, which may be used with Tenable.io Vulnerability Management to detect the vulnerability passively on the network. Tenable.io Container Security has also been updated to detect the Exim off-by-one RCE vulnerability in Docker container images. The following table summarizes Tenable’s coverage.

Cisco has updated the advisory several times since the initial release to reflect the affected vectors as well as to make corrections to the patch versions. Tenable continues to monitor this situation and update our coverage as necessary.

What should you do?

If you’re running a version of Exim prior to 4.90.1, make sure you update to the most current release. Exim notes that all versions of Exim prior to 4.90.1 are now obsolete and that 3.x releases are also obsolete and should not be used.

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